The release of larvae by female lobsters is confined to a few minutes at night, and is repeated at a similar time on each night over several weeks. The rhythm is shown to be partly controlled by an endogenous component in the female. Eggs in vitro have an exogenous, light-stimulated hatching rhythm, but are arhythmic in continuous light or darkness. In light/dark regimes the time of hatching is not influenced by daylength, but occurs sooner after sunset as temperature increases.The time when females perform their part of the hatching process in light/dark regimes is shown to depend on temperature, daylength and previous photoperiodic experience. A theoretical equation linking these variables is proposed and tested. In continuous darkness, the rhythm free-runs; for part of the year, the period of the rhythm is effectively independent of temperature, but for the remainder of the year, the length of the period is markedly temperature-dependent. 21MBI 58 640 J. R. BRANFORD long-term photoperiodic experience. The hatching of detached eggs, isolated from females, was also studied.

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